I bought this recently, having watched dragons den a while ago thinking 'it doesnt look like a bad idea'.
Firstly, it does exactly what it says on the tin. It holds 500ml of water (the same size as a standard shop bought water bottle), and folds up to about 1/3 of its original size. It looks ok and it can fit into a smaller space than a normal bottle can when empty.
On the website there was a lot of stuff about saving the planet and apps to find public water sources etc. These are pretty much a sideshow as this product is not going to stop people buying bottled water, and I am not going to carry it round looking for places to fill it up. I have personally found it handy over a regular sized bottle as I can fold it up and put it in my pocket with my membership card, house-key and a small rolled up towel and jog down to the gym to do weights. This way I avoid having to get changed or buy locker tokens, and I get more time to do exercise I like alongside a cardio workout, without having to spend all my time being bored on the treadmill.
This product does have some negative points though. Firstly, being collapsible the materials are not as sturdy as I imagined they would be. This is not a dealbreaker though, and they do the job well enough. Secondly the bottle does tend to pop out at first, so I would recommend taking a few minutes and going around each fold and pressing them down (which seems to have worked). Thirdly, the opening is fairly large and can scrape your nose, but I just tip the water into my mouth anyway so it hasnt bothered me. If I take a bag to the gym I would take my regular water bottle along, the collapsible one is no better unless there is a shortage of space.
A decent enough product and at under £5 it is very decent considering most people will spend that on a sports bottle anyway. Probably not worth it for everyone, but it does give me a bit more flexibility in the way I choose to exercise.